Concept Map =
- Tenth Standart Science Textbook
- Tenth Standard Ready Lesson CD from Education Department – Science Subject
Concept #1 Electro magnetic Induction
- Recall the the properties of magnets
- Relation between magnet and electricity
- How emf induced
- Emf depends upon the number of turns and speed of the magnet
- Michel faraday experiment
Notes for teachers
- Electromagnetic Induction
We have seen previously that when a DC current pass through a long straight conductor a magnetising force, H and a static magnetic field, B is developed around the wire. If the wire is then wound into a coil, the magnetic field is greatly intensified producing a static magnetic field around itself forming the shape of a bar magnet giving a distinct North and South pole.
- air cored electromagnetic coil
Air-core Hollow Coil The magnetic flux developed around the coil being proportional to the amount of current flowing in the coils windings as shown. If additional layers of wire are wound upon the same coil with the same current flowing through them, the static magnetic field strength would be increased.
Therefore, the Magnetic Field Strength of a coil is determined by the ampere turns of the coil. With more turns of wire within the coil the greater will be the strength of the static magnetic field around it.
But what if we reversed this idea by disconnecting the electrical current from the coil and instead of a hollow core we placed a bar magnet inside the core of the coil of wire. By moving this bar magnet “in” and “out” of the coil a current would be induced into the coil by the physical movement of the magnetic flux inside it.
Likewise, if we kept the bar magnet stationary and moved the coil back and forth within the magnetic field an electric current would be induced in the coil. Then by either moving the wire or changing the magnetic field we can induce a voltage and current within the coil and this process is known as Electromagnetic Induction and is the basic principal of operation of transformers, motors and generators.
Electromagnetic Induction was first discovered way back in the 1830’s by Michael Faraday. Faraday noticed that when he moved a permanent magnet in and out of a coil or a single loop of wire it induced an ElectroMotive Force or emf, in other words a Voltage, and therefore a current was produced.
So what Michael Faraday discovered was a way of producing an electrical current in a circuit by using only the force of a magnetic field and not batteries. This then lead to a very important law linking electricity with magnetism, Faraday’s Law of Electromagnetic Induction. So how does this work?.
When the magnet shown below is moved “towards” the coil, the pointer or needle of the Galvanometer, which is basically a very sensitive centre zero’ed moving-coil ammeter, will deflect away from its centre position in one direction only. When the magnet stops moving and is held stationary with regards to the coil the needle of the galvanometer returns back to zero as there is no physical movement of the magnetic field.
Likwwise, when the magnet is moved “away” from the coil in the other direction, the needle of the galvanometer deflects in the opposite direction with regards to the first indicating a change in polarity. Then by moving the magnet back and forth towards the coil the needle of the galvanometer will deflect left or right, positive or negative, relative to the directional motion of the magnet.
Faraday’s Law of Induction From the above description we can say that a relationship exists between an electrical voltage and a changing magnetic field to which Michael Faraday’s famous law of electromagnetic induction states: “that a voltage is induced in a circuit whenever relative motion exists between a conductor and a magnetic field and that the magnitude of this voltage is proportional to the rate of change of the flux”.
In other words, Electromagnetic Induction is the process of using magnetic fields to produce voltage, and in a closed circuit, a current.
So how much voltage (emf) can be induced into the coil using just magnetism. Well this is determined by the following 3 different factors.
1). Increasing the number of turns of wire in the coil. – By increasing the amount of individual conductors cutting through the magnetic field, the amount of induced emf produced will be the sum of all the individual loops of the coil, so if there are 20 turns in the coil there will be 20 times more induced emf than in one piece of wire. 2). Increasing the speed of the relative motion between the coil and the magnet. – If the same coil of wire passed through the same magnetic field but its speed or velocity is increased, the wire will cut the lines of flux at a faster rate so more induced emf would be produced. 3). Increasing the strength of the magnetic field. – If the same coil of wire is moved at the same speed through a stronger magnetic field, there will be more emf produced because there are more lines of force to cut. If we were able to move the magnet in the diagram above in and out of the coil at a constant speed and distance without stopping we would generate a continuously induced voltage that would alternate between one positive polarity and a negative polarity producing an alternating or AC output voltage and this is the basic principal of how a Generator works similar to those used in dynamos and car alternators.
In small generators such as a bicycle dynamo, a small permanent magnet is rotated by the action of the bicycle wheel inside a fixed coil. Alternatively, an electromagnet powered by a fixed DC voltage can be made to rotate inside a fixed coil, such as in large power generators producing in both cases an alternating current.
- Activity 1 - Give the magnets to students and check the properties of magnets. Ask the students properties of magnets. Ask magnetic lines of force using phet tool of magnets and electromagnet.
- Activity 2 - show the different copper coils and electromagnet experiment to students and ask is it possible get electricity from magnet
- Activity 3 - Show the electromagnetic induction using galvanometer and copper coil. Observe the changes of galvanometer deflections.
- Activity 4 - Show the short video of number of coils increasing emf is increasing.
- Activity 5 - Show the video of Michel Faraday experiment.
Assessment Activities for CCE
Formative Assesment Questions
1.If a magnetic needle placed along the north and south pole of a bar magnet as shown in the figure, what is the polarity of the a b c and d. mark 2
- Pole of a ________
- pole of b _______
- Pole of c _________
- Pole of d _________
2.The device that detects the induced emf is called _________________ mark 1
3.Which the following emf is produced? Yes/no
a) Coil is steady, magnet is moved inside the coil _______________
b) Coil and magnet are moved away in opposite direction _______________
c) Magnet is steady, Coil is moved towards magnet ____________
d) Coil and magnet both are moved together __________________ mark 2
4.copper coil is connected to Galvanometer and a bar magnet. What are the changes in the following
- Magnet kept inside the coil ____________________________
- Magnet is pushed slowly ________________________________
- Magnet is pushed inside the coil very quickly _____________________ mark 2
5. Which the following coil is more emf produced when magnet movement inside the coil? mark 1
6. A copper coil, a strong bar magnet and a led bulb are on the table. How do you glow on the led bulb? Mark 2
Record Grade List
Concept #2 -
Notes for teachers
Concept #3 -
Notes for teachers
- Activity 1 -